1. Field of the Invention
The invention is related to a logging-type cable, i.e., a cable that goes in and out of a well repeatedly, that has one fiber directly exposed to the fluids in the well bore and one that is not exposed and is strain free. More particularly, it is related to a high pressure high temperature (HPHT) logging cable.
2. Background
Fiber optic sensing for oil and gas wells is often done with logging cables. These cables typically have a sensing tool attached to the end of the cable as it is lowered into the well. The tool can measure a number of features of the well—physical, chemical and structural properties of the well. The measurements are taken specifically where the tool is located and are either transferred back to the surface via a copper or fiber optic link or may be stored in memory in the tool itself until it can be retrieved. At times, optical fiber can be used in the logging cable and is typically encased in a hermetic stainless steel tube. As mentioned, this fiber can be simply for data transport from the surface to the tool and back. The optical fiber can also be used as a sensor itself to detect temperature along the length of the logging cable using distributed temperature sensing. Ideally, the sensing of various parameters in the well would all be detected along the entire length of the logging cable instead of just at the tool as this would provide the operator of the well added information to assist in maximizing the performance of the well. Sensing technologies now exist to measure the pressure along the length of an optical fiber similar to the way temperature is measured along the length of the optical fiber. The key issue is that the fiber must be exposed to the well bore fluids along its entire length. As an optical fiber is relatively fragile in comparison to the logging cable, the cable design must find the right balance of having the fiber exposed to the well bore fluids yet be protected adequately to ensure the fiber operation is not disrupted due to fiber damage.